1. Field of the Invention
The present invention relates to a sheet conveyance technique in a printing apparatus having a printhead.
2. Description of the Related Art
In recent years, a printing apparatus is expected to increase the speed of printing to improve the productivity. As one of methods of increasing the speed, an interval between printing sheets to be continuously fed is shortened. As a technique of shortening the interval between printing sheets, in addition to a method of simply shortening the interval between a preceding sheet and a succeeding sheet, there is provided a method of conveying sheets by making the marginal area of the leading edge of the succeeding sheet overlap the marginal area of the trailing edge of the preceding sheet, and forming images while the sheets overlap each other (see Japanese Patent Laid-Open No. 2001-324844). This means that images are formed by excluding unnecessary portions (the interval between printing sheets and the marginal portion of each printing sheet) as much as possible except for image forming areas. This is a very effective method to increase the speed of printing.
However, if an inkjet printing apparatus executes high-density printing on an area where printing sheets overlap each other using a large amount of ink, wavy wrinkles called cockling can occur on the printing sheet due to moisture of the ink. For example, when a partial area of the leading edge of the succeeding sheet is made to overlap the marginal area of the trailing edge of the preceding sheet, the reverse surface of the sheet is constrained by a flat plate. When, therefore, cockling occurs, the printing sheet may unwantedly float, and graze against the printhead, thereby causing a stain on the printing sheet, or disabling conveyance to a discharge roller or the like to cause a sheet jam. In addition, when the distance between the printhead and the obverse surface of the printing sheet becomes unstable, an ink landing position may shift to degrade the image quality.